1. Field of the Invention
The invention relates generally to monolithically fabricated micromachined structures and, more particularly, to micromachined structures in which a first frame is coupled to a plate or to a second frame for rotation of the plate or second frame with respect to the first frame about an axis.
2. Description of the Prior Art
A fundamental micromachined structure having many diverse uses is a torsional oscillator formed by a first frame that is coupled to a plate or to a second frame by diametrically opposed torsion bars that extend between the first frame and the plate or second frame. The torsion bars permit rotation of the second frame or the plate with respect to the first frame about an axis established by the torsion bars. Practical uses for this basic micromachined structure include optical beam micromachined torsional scanners having a reflective surface, described in U.S. Pat. No. 5,629,790 (xe2x80x9cthe ""790 patentxe2x80x9d), that have uses in digital imaging, bar code reading and printing as described in U.S. Pat. No. 5,841,553 (xe2x80x9cthe ""553 patentxe2x80x9d), and in magneto-optical recording as described in Published Patent Cooperation Treaty (xe2x80x9cPCTxe2x80x9d) International Patent Application WO 98/09289 entitled xe2x80x9cOptical head Using Micro-Machined Elementsxe2x80x9d (xe2x80x9cthe ""09289 PCT patent applicationxe2x80x9d). Other practical applications for the basic micromachined structure occur in various other scientific and industrial systems such as rate gyroscopes described in U.S. Pat. No. 5,488,862, micro-flow meters described in U.S. Pat. No. 5,895,866, and profilometer and/or atomic force microscope (xe2x80x9cAFMxe2x80x9d) heads described in U.S. Pat. No. 5,861,549 that are used in surface inspection systems.
Collectively, the preceding patents describe various techniques for applying electrostatic and electromagnetic forces to the plate and/or second frame to energize rotation about the axis established by the torsion bars. The usefulness of the basic micromachined structure is greatly enhanced by integrating a torsion sensor into at least one of the torsion bars as described in U.S. Pat. No. 5,648,618 (xe2x80x9cthe ""618 patentxe2x80x9d) for measuring rotation of the second frame or the plate with respect to the first frame about an axis established by the torsion bars.
FIG. 1 illustrates a torsional oscillator, i.e. a typical torsional scanner, such as that described in ""790 patent which is referred to by the general reference character 52. The torsional scanner 52 includes torsion bars 54 which extend inward from an encircling frame 56 to support a torsional scanner plate 58 and permit the plate 58 to rotate about an axis 62 established by the torsion bars 54. The frame 56 rests upon an insulating substrate 64 which also carries a pair of electrically conductive electrodes 66. A frame shaped spacer 68, resting on the frame 56, supports a membrane window 72 a short distance above the plate 58. A light beam 74, indicated by arrowed lines in FIG. 1, enters the torsional scanner 52 through the membrane window 72, impinges upon and reflects from a mirror surface 76 on the plate 58, and then exits the torsional scanner 52 through the membrane window 72. A voltage V applied alternatively between the plate 58 and first one and then the other of the electrodes 66 that switches back and forth between the electrodes 66 at the frequency of the principal torsional vibrational mode of the plate 58 applies an electrostatic force to the plate 58 which urges it to rotate back and forth at that frequency about the axis 62.
When using the basic micromachined structure for the optical beam torsional scanners 52, a mirror surface 76 on the plate 58 or second frame deflects the light beam 74, usually from a fixed light source, over an angle ranging from several degrees to tens of degrees. Such reflective torsional scanners 52 may be used for sweeping a beam of light back-and-forth at a frequency determined in part by a mechanical resonant frequency of the plate 58 or second frame. Alternatively, torsional scanners 52 may be used for moving or switching a point at which a beam of light impinges upon one or more other elements between two (2) or more alternative locations.
The ""790 patent describes a critical mechanical vibrational mode spectrum which commercially practical torsional oscillators should possess. This mode spectrum is particularly desirable for sinusoidal oscillation of the torsional scanner 52 at video or even higher frequencies. The same mode spectrum is also advantageous when the torsional scanner 52 operates in a quasi-static mode such as when switching a point at which the light beam 74 impinges upon other elements. Operating in a quasi-static mode, the torsional scanner 52 rotates to and remains fixed in one orientation for some interval of time, and subsequently rotates swiftly through a relatively large angle to another orientation where it again remains fixed for some interval of time.
As illustrated in FIG. 2 of U.S. Pat. No. 5,673,139 (xe2x80x9cthe ""139 patentxe2x80x9d), for applications in which torsional scanners 52 must rotate about one or two axes and must be packed very closely together it is often desirable to eliminate open space between the frame 56 and the plate 58 or second frame occupied by the length of the torsion bars 54. This open space may be eliminated if the length of the torsion bars 54 is located within a xe2x80x9cbutterfly-shapedxe2x80x9d frame as illustrated in the ""139 patent, or within a butterfly-shaped plate 58. However, since torsion bars 54 tend to be very long and slender even the butterfly-shaped plate 58 or frame such as that illustrated in the ""139 patent may occupy too much space. Merely shortening the torsion bars 54 to reduce the space which they occupy can be disadvantageous because, in general, shortening the torsion bars 54 make them stiffer which raises the frequency of the principal torsional vibrational mode, or alternatively increases the force that must be applied to rotate either the plate 58 or the second frame about the axis 62.
In many instances for various reasons energizing rotation of the plate 58 with low power electrostatic fields as described above is highly desirable. However, some applications for the torsional scanner 52 may require that the plate 58 rotate through large angles. Due to an electrostatic instability, using electrostatic force to energize rotation of the plate 58, or a second frame, either statically or dynamically without feeding a signal that is proportional to angular rotation back to the circuit that generates the electrostatic drive signals generally limits the rotation angle of the plate 58.
The electrostatic instability occurs because a restoring torque applied to the plate 58 by the torsion bars 54 increases linearly with rotation of the plate 58 while a driving torque generated by electrostatic attraction between the plate 58 and one of the electrodes 66 increases quadratically as the separation between them decreases. For sinusoidally oscillating electrostatically driven torsional scanners operating at the resonance frequency of their principal torsional vibrational mode, the electrostatic instability is of little concern because voltage applied between the electrodes 66 and the plate 58 is generally zero (0.0) when the plate 58 rotates nearest to the closest electrode 66. That is, for sinusoidally oscillating electrostatically driven torsional scanners operating at the resonance frequency of their principal torsional vibrational mode, rotation of the plate 58 is out of phase with, i.e. lags, application of the alternating voltage V between the plate 58 and first one and then the other of the electrodes 66. However, if for quasi-static operation a constant voltage V that exceeds some threshold value were applied across the plate 58 and one of the electrodes 66, rotation of the plate 58 about the axis 62 becomes unstable. That is, if the voltage V applied between the plate 58 and one of the electrodes 66 has a particular value and responsive to that voltage the plate 58 rotates to a particular angle, unless restrained mechanically the plate 58 will continue rotating to a position nearest to the electrode 66 without any change in the voltage V.
The curves in FIG. 2 graphically illustrate the phenomenon of electrostatic instability. The straight line 82 in FIG. 2, which slopes upward from left to right proportional to a torsional spring constant for the torsion bars 54, indicates the amount of restoring torque that the torsion bars 54 apply to the plate 58 upon its rotation about the axis 62 to various angular orientations. A family of driving torque curves 84a, 84b and 84c in FIG. 2 depict various driving torques applied to the plate 58 by increasingly higher fixed voltages Va, Vb and Vc between the plate 58 and the electrode 66 for various angles of rotation of the plate 58 about the axis 62. The electrostatic attractive driving torque for a particular voltage Va or Vb in relation to the restoring torque illustrated by the straight line 82 are in equilibrium where driving torque curves 84a and 84b respectively intersect the straight line 82 at points labeled Aa and Ab. The first intersection point Aa for the driving torque curve 84a is a point of stable equilibrium at which further rotation of the plate 58 produces a larger restoring torque than the increase in driving torque applied to the plate 58 electrostatically. A second intersection point Ba is a point of unstable equilibrium because any increase in the rotation angle of the plate 58 increases the electrostatic driving torque more rapidly than the restoring torque. Therefore, if for the voltage represented by the driving torque curve 84a the plate 58 rotates to the angle at which the straight line 82 and driving torque curve 84a intersect at Ba, then unless restrained mechanically the plate 58 will continue rotating to a position nearest to the electrode 66 without any change in the voltage V.
If the fixed voltage V increases, for example from the driving torque curve 84a to the driving torque curve 84b, the driving torque applied electrostatically increases and the two points of intersection move closer together to the points Ab and Bb. However, as the voltage V applied across the plate 58 and the electrodes 66 continues increasing eventually the curves for restoring torque and driving torque become tangent. When tangency occurs, a stable point of intersection no longer exists and application of a fixed voltage V of that magnitude causes the plate 58 to instantaneously flip and slam into the underlying electrode 66. This situation is illustrated by the driving torque curve 84c for which the two intersection points coincide at a single point AcBc. Consequently, for the voltage V illustrated by the driving torque curve 84c the plate 58 is no longer statically stable without feeding back a signal that is proportional to angular rotation to the circuit that generates the electrostatic drive signals. Consequently, without angular rotation feedback quasistatic rotation of the plate 58 cannot be controlled for many desirable angular orientations of the plate 58.
Due to this electrostatic instability, electrostatically energized rotation of the plate 58 about the axis 62 is typically limited to approximately one-third (⅓) of the separation between the rest position of the plate 58 and the electrode 66. For a particular size of plate 58, obtaining an appreciable angle of rotation without feeding back an angular rotation signal to the electrostatic drive circuit requires increasing the spacing between the plate 58 and the electrodes 66. However, wider spacing between the plate 58 and the electrodes 66 requires applying a higher driving voltage V across the plate 58 and the electrode 66. However, the extent to which the voltage V applied between the plate 58 and the electrodes 66 may increase is limited because that voltage cannot exceed the breakdown voltage between them. Alternatively, increasing the torsional spring constant substantially in regions of FIG. 2 where the electrostatic instability occurs provides a stable equilibrium for larger angular rotations of the plate 58 about the axis 62 without increasing the spacing between the plate 58 and the electrodes 66.
The ""09289 PCT patent application depicts and describes a torsional scanner 52 having a non-linear torsional spring constant. As illustrated in FIG. 3 hereof and in FIG. 3 of the ""09289 PCT patent application, the torsional spring constant for the torsion bars 54 disclosed in the ""09289 PCT patent application is rendered non-linear by attaching one or more tethers 86 to the plate 58. The tethers 86 consist of springs made of nitride or oxide that have corrugations oriented parallel with the axis 62 established by the torsion bars 54. As the plate 58 rotates about the axis 62, the tethers 86 initially increase the torsional spring constant of the torsion bars 54 only slightly. However as the plate 58 rotates further about the axis 62 the torque which the tethers 86 apply increases rapidly thereby creating a non-linear torsional spring constant for the torsional scanner 52 depicted in FIG. 3.
FIG. 3a illustrates a driving torque curve 92 for application of a fixed voltage V between the plate 58 and one of the electrodes 66 depicted in FIG. 3. A first straight line segment 94a in FIG. 3a illustrates a hypothetical restoring torque applied only by the torsion bars 54 upon initial rotation of the plate 58 about the axis 62 up to a critical angular orientation. A second straight line segment 94b in FIG. 3a illustrates the restoring torque that the torsion bars 54 together with the tethers 86 hypothetically apply upon rotation of the plate 58 about the axis 62 beyond the critical angular orientation. The differing slopes of the two line segments 94a and 94b in FIG. 3a depict a torsional spring constant that changes from k1 to k2 at the critical angular orientation due to the restraint which the tethers 86 apply to the plate 58. A low torsional spring constant k1 permits good initial rotation of the plate 58 about the axis 62. The change from the torsional spring constant k1 to the torsional spring constant k2 eliminates the electrostatic instability for larger angular rotations of the plate 58 about the axis 62. However, as those skilled in the art will recognize, in reality the tethers 86 do not actually produce the abrupt change from a flexible to a stiff torsional spring constant suggested by FIG. 3a. 
Nevertheless, for the torsional scanner 52 depicted in FIG. 3, if the torsional spring constant of the torsion bars 54 is small in comparison with the effect of the tethers 86 on the spring constant, then rotation of the plate 58 may be dominated by the tethers 86. Moreover, if the tethers 86 do not restrain the plate 58 exactly symmetrically, they tend to bend it thus destroying its optical flatness. As noted in the ""09289 PCT patent application, the torsional scanner 52 including the tethers 86 depicted in FIG. 3 permits stable rotation of the plate 58 about the axis 62 only up to angles of plus or minus two degrees (∓2.0xc2x0) because the tethers 86 produce an extreme non-linearity in the torsional spring constant.
The ""790 patent discloses advantages that inclusion of a box frame reinforcing rim around the plate 58, or the second frame, provides for the torsional scanner 52. The box frame reinforcing rim thickens the plate 58, or the second frame, about their periphery while leaving the remainder of their structure thin. The box frame reinforcing rim maintains the plate 58 optically flat, and also provides differing thicknesses for the torsion bars 54 and the frame 56 thereby increasing the rigidity of the torsional scanner 52. In comparison with a solid plate 58 or second frame, reinforcing the plate 58, or the second frame, with a box frame also reduces the mass of the plate 58, or the second frame, while preserving its moment of inertia. A large moment of inertia increases the Q of the torsional scanner 52 as illustrated by the analysis of Buser, et al. (Sens. and Act., A23, 1990, pg. 323).
A major concern in fabricating reflective torsional scanners 52 is the reflectivity and planarity of the mirror surface 76 throughout a range of operating temperatures. Increasing the reflectivity of or controlling the polarization of light reflected from the plate 58 may also require depositing dielectric coatings over the mirror surface 76. Usually inorganic materials such as oxides, nitrides etc. deposited onto a metal coated mirror surface 76 yield the desired reflective properties. Because such inorganic material coatings must be at least 0.1 to 0.2 microns thick, the stress which they may apply to the plate 58 is a major concern, particularly since they must be deposited onto torsional scanners only a few microns thick.
For certain applications, torsional oscillators must dissipate a significant amount of heat from the plate 58. For example, if rotation of the plate 58 with respect to the frame 56 about the axis 62 is energized electro-magnetically using a coil attached to the plate 58, then the plate 58 must dissipate heat generated by an electric current flowing through the coil, i.e. must dissipate i2R heating. However, even if the plate 58 does not carry a coil because electrostatic force energizes rotation, reflecting a 100 milliwatt (mw) light beam 74 from a mirror surface 76 that is ninety-eight and one-half percent (98.5%) reflective, requires that the plate 58 must dissipate 1.5 mw of energy deposited there by the incident light beam 74. Absorption of 1.5 mw of energy into 1.0 mm2 of silicon, a relatively low thermal conductivity material, may raise the temperature of the plate 58 by twenty (20.0) to thirty (30.0) xc2x0C. above the surrounding ambient temperature. The temperature of the plate 58 increases even more dramatically if the plate 58 has poor thermal conductivity to the remainder of the torsional scanner 52.
In many applications for torsional scanners 52 such as two dimensional (xe2x80x9c2Dxe2x80x9d) pointing or scanning, sometimes several electrical leads must pass from the frame 56 to the plate 58 via the torsion bar 54 that includes a torsion sensor such as that as described in the ""618 patent. The locations for these numerous electrical leads can be severely constrained by the width of the torsion bar 54. Extremely narrow torsion bars 54 can also constrain placement of electrical leads and operation of a torsion sensor.
Various applications for reflective micromachined torsional scanners 52, such as fiber optic switching, envision using the torsional scanner 52 for optically aligning a beam of light using stationary or quasi-stationary positioning along at least one axis of a 2D raster. Another non-parallel axis of rotation for the plate 58 may provide either periodic motion (sinusoidal, linear), or static or quasi-statically positioning for the light beam. Such applications may result in effectively switching a beam of light on or off by flipping the mirror surface 76 into the light beam""s path. Usually for such applications it is desirable to flip the plate 58 into the light beam as swiftly as practicable. Further-more, after the mirror surface 76 intersects the light beam 74, making small trimming adjustments in, i.e. tailoring, the angle at which the mirror surface 76 reflects the light beam may be advantageous or even required.
In fiber optic switching technologies, such compound motions of the plate 58 may provide tracking along one or two axes to keep a light beam on target. Such pointing applications may require that the light beam be deflected along at least one axis through a relatively large angle, and then held at a particular angle for an extended interval of time while dissipating a small amount of power in the torsional scanner 52. For such quasi-static pointing applications the plate 58 must be held stationary after rotating through a very large angle, e.g. 5xc2x0 to 45xc2x0. Such large angular rotations are difficult to achieve electrostatically, because, as explained previously, large electrostatically energized rotations of the plate 58 inherently require a very large spacing between the plate 58 and the electrodes 66. Such large angular rotations are also difficult to achieve electro-magnetically because the magnetic field generated by a coil carried on the plate 58 becomes reoriented with respect to an external unidirectional magnetic field as the plate 58 rotates about the axis 62. Consequently, electro-magnetically energized quasi-static rotation of the plate 58 through large angles normally requires that the coil carry very large electrical current while holding the plate 58 at a fixed angular rotation. As described above, such a large electrical current significantly raises the temperature of the plate 58.
For some applications of torsional oscillator, as disclosed in the ""139 and ""790 patents it is advantageous to include one or more light detecting elements, usually as photo-diodes, in the torsional scanner 52, perhaps locating them on the plate 58. As described in U.S. Pat. No. 5,416,324 (xe2x80x9cthe ""324 patentxe2x80x9d), including a polarized light detector is particularly advantageous for some image processing applications. Similarly, some application for torsional oscillators will likely require detecting Faraday rotation of a planar polarized light for sensing the presence or absence of magnetically recorded data. The ""324 patent discloses a two dimensional (xe2x80x9c2Dxe2x80x9d) ensemble of receiver assemblies. Each of the receiver assemblies includes four (4) light detectors each of which responds to light polarized in a different orientation. The ""324 patent discloses that such polarizers may be formed by wire grids disposed immediately adjacent to the light detector, or at an appropriate location in the optical system. The cost of the imaging system disclosed in the ""324 patent could be significantly reduced if it were possible to reduce the ensemble of receiver assemblies to a single receiver assembly.
The ""790, ""553, ""618, ""139 and ""324 patents, and the ""09289 PCT patent application are hereby incorporated by reference.
An object of the present invention is to provide improved structures for micromachined members coupled for relative rotation by torsional flexure hinges.
Another object of the present invention is to improve operating characteristics of micromachined members coupled for relative rotation by torsional flexure hinges.
Another object of the present invention is to provide torsional flexure hinges for micromachined members coupled for relative rotation that are more compact than conventional, unfolded torsion bars.
Another object of the present invention is to provide torsional flexure hinges for micromachined members coupled for relative rotation which exhibit increased separation between a frequency of a member""s principal torsional vibrational mode and frequencies of that member""s other vibrational modes.
Another object of the present invention is to provide structures for micromachined members coupled for relative rotation by torsional flexure hinges for which torque required to rotate a member about an axis established by the torsional flexure hinges increases non-linearly with increasing angular rotation of the micromachined members.
Another object of the present invention is to provide an electrostatic drive circuit for applying a drive signal that urges micromachined members coupled for relative rotation by torsional flexure hinges to rotate about an axis established by the torsional flexure hinges which changes non-linearly with angular deflection of the micromachined members.
Another object of the present invention is to decrease the electrostatic potential required for rotating micromachined members coupled for relative rotation by torsional flexure hinges.
Another object of the present invention is to provide structures for micromachined members coupled for relative rotation by torsional flexure hinges having enhanced thermal conductivity.
Another object of the present invention is to ruggedize torsional flexure hinges that couple micromachined members for relative rotation which include a torsion sensor.
Another object of the present invention is to provide a method for enhancing optical reflectivity of micromachined members which does not alter the micromachined member""s flatness.
Another object of the present invention is to provide structures for optically reflective micromachined members which permit tailoring reflective characteristics of a mirror surface.
Another object of the present invention is to provide a micromachined member which includes a polarization-sensitive scanned photo-detector.
Another object of the present invention is to provide structures for micromachined members coupled for relative rotation by torsional flexure hinges together with an electronic drive therefor that urges members to rotate about an axis established by the torsional flexure hinges swiftly and then immediately fixes the micromachined member at a specified angular rotation.
Another object of the present invention is to provide structures for micromachined members coupled for relative rotation by torsional flexure hinges together with a lower power consumption electronic drive therefor that urges members to rotate about an axis of the torsional flexure hinges swiftly and then immediately fixes the micromachined member in a specified angular orientation.
Another object of the present invention is to provide structures for micromachined members coupled for relative rotation by torsional flexure hinges together with a lower power consumption electronic drive therefor that permits trimming the orientation of a member after that member has been rotated through a specified angle and been fixed at that orientation.
Briefly, the present invention includes an improved integrated, micromachined structure that has a reference member, a pair of diametrically opposed torsional flexure hinges projecting from the reference member, and a dynamic member supported by the pair of torsional flexure hinges from the reference member. The torsional flexure hinges support the dynamic member from the reference member for rotation about an axis established by the pair of torsional flexure hinges. As used herein, the phrase torsional flexure hinge, when applied most broadly, encompasses a conventional unfolded torsion bar, and also encompasses hinge structures in which one or more hinge segments included in that structure do not experience pure torsion when the dynamic member rotates about the axis established by the torsional flexure hinges. The reference member, the pair of torsional flexure hinges and the dynamic member are all monolithically fabricated using a stress-free semiconductor layer of a silicon substrate. The integrated micromachined structure also includes drive means for imparting rotary motion to the dynamic member about the axis established by the pair of torsional flexure hinges. The drive means may apply torque electrostatically or electro-magnetically to the dynamic member, either singly or in combination.
The improved micromachined structure in one embodiment forms at least one of the torsional flexure hinges by coupling together first ends of at least three torsion-bar segments. The first end of each torsion-bar segment is located along the multi-segment torsional flexure hinge between the reference member and the dynamic member. In another embodiment, the improved micromachined structure forms at least one of the torsional flexure hinges by disposing a bifilar beam between the reference member and the dynamic member. The bifilar beam is disposed symmetrically on opposite sides of the axis for rotation of the dynamic member established by the pair of torsional flexure hinges.
Another embodiment of the improved micromachined structure that enhances electrostatic stability includes an appendage having a first end that attaches to one of the torsional flexure hinges at a point along the torsional flexure hinge that is located between the reference member and the dynamic member. The appendage projects outward from the torsional flexure hinge and is shaped so that upon sufficient rotation of the dynamic member about the axis with respect to the reference member a projecting end of the appendage contacts a stop having a fixed relationship with the reference member. When the projecting end of the appendage contacts the stop the torsional spring constant of the torsional flexure hinge changes. In another embodiment the improved micromachined structure includes a tether that is coupled at a first end to the reference member and at a second end to one of the torsional flexure hinges at a point along the torsional flexure hinge that is located between the reference member and the dynamic member. Upon sufficient rotation of the dynamic member about the axis with respect to the reference member the tether changes the torsional spring constant of the torsional flexure hinge. In yet another electrostatically energized embodiment, the improved micromachined structure includes a torsion sensor that is adapted for producing a signal responsive to angular rotation of the dynamic member about the axis with respect to the reference member. The signal produced by the torsion sensor is fed back to the drive means for altering the drive signal which electrostatically energizes rotation of the dynamic member.
An improved torsional oscillator increases torque applied electrostatically between an electrode and a dynamic member that includes a reinforcing rim by sharpening a tip of the reinforcing rim immediately adjacent to the electrode which enhances the electric field between them. In another improved torsional oscillator, to enhance the Q of the torsional oscillator a hollow first cavity in the dynamic member, that is encircled by the reinforcing rim, is disposed adjacent to a hollow second cavity that is formed in a substrate and which opens toward the first cavity formed into the dynamic member. In another improved torsional scanner either the reference member or the dynamic member includes a slot formed therein that is disposed alongside one of the torsional flexure hinges. Damping material disposed across the slot and contacting the adjacent torsional flexure hinge near the reference member reduces the torsional oscillator""s Q. Yet another improved torsional oscillator adds auxiliary driving-plates along opposite sides of the torsional flexure hinges between the dynamic member and the reference member. The auxiliary driving-plates are coupled to the torsional flexure hinge adjacent to the dynamic member and have a combined width perpendicular to the rotation axis which is less than a width of the dynamic member perpendicular thereto. In this improved torsional oscillator, the drive means applies an electrostatic drive signal between the auxiliary driving-plates and to electrodes disposed adjacent thereto.
In another improved torsional oscillator, the torsional flexure hinges have a width-to-thickness (w:t) ratio that exceeds four-to-one (4:1) to increase thermal conductivity between the dynamic member and the reference member in comparison with narrower and thinner torsional flexure hinges having an equivalent torsional spring constant. Further increasing the width-to-length (w:1) ratio of the torsional flexure hinges to greater than one-to-two (1:2) provides a non-linear torsional spring constant that improves electrostatic stability. Another improved torsional oscillator improves thermal conductivity between the dynamic member and the reference member by fabricating them from isotopically pure 14Si28 silicon.
In another improved torsional oscillator the dynamic member includes a stress relief cut that almost completely encircles a decoupled portion of the dynamic member. The stress relief cut establishes beams for supporting that decoupled portion of the dynamic member from a surrounding portion thereof whereby stress is decoupled between the decoupled portion and the surrounding portion. In another improved torsional scanner both front and back sides of the dynamic member have a reflective mirror coating applied thereto to balance any stress applied to the dynamic member. In another improved torsional oscillator at least one of the torsional flexure hinges includes a widened section having a torsion sensor disposed there which produces a signal responsive to angular rotation of the dynamic member about the axis with respect to the reference member.
In another improved torsional scanner adapted for switching a light beam the drive means initially energizes rotation of the dynamic member about the axis electro-magnetically with a current pulse. The current pulse impulsively starts the dynamic member rotating about the axis established by the torsional flexure hinges. After the dynamic member rotates near to a pre-established orientation, the drive means holds the dynamic member in the pre-established orientation with an electrostatic force.
In another improved torsional oscillator the dynamic member carries a wire grid polarizer disposed adjacent to a photo-detector so that illumination incident on the photo-detector must traverse the wire grid polarizer before impinging upon the photo-detector.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.